(a) Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to a power supply of an LCD and voltage sequence control methods that enhance the LCD panel performance and prevent a latch up by applying stabilized voltages to the gate driver integrated circuits (ICs). The voltage sequence is determined by the turn-off voltage level among voltage levels applied to a gate driver IC that outputs a driving voltage for the panel.
(b) Description of the Related Art
Generally, LCD devices include an LCD panel where a liquid is injected between the two glass substrates on which pixels and electrodes are formed, a printed circuit board (PCB) where various integrated circuits for driving the LCD panel are mounted and interfacing with the electrode of the LCD panel, a back light unit providing the display with required light, a power supply providing various driving voltages, and an assembly of mold frames or chassis.
A predetermined voltage level applied to the LCD panel, operates a thin film transistor (TFT) that constitutes each pixel to display a certain image.
Controlling the voltage applied to display the desired image is considerably important in LCD display technologies. U.S. Pat. No. 5,777,611 discloses an apparatus for controlling power sequence of an LCD module.
Specifically, a plurality of voltages are applied in a predetermined sequence to a plurality of gate driver ICs mounted on the PCB, and each gate driver IC outputs a voltage to drive the LCD panel.
In a conventional method, as shown in FIG. 1, a turn-on voltage Von with approximately 20V and a turn-off voltage Voff with approximately xe2x88x927V are applied to a driver IC 3. Such voltages are applied or removed in accordance with a predetermined sequence. An incorrectly controlled sequence causes a latch up, which may result in a failure in driving the LCD panel. Here the driver IC 3 is a gate driver IC.
Voltages Von and Voff are generated by applying a constant voltage VDD to a DC-to-DC converter.
Generally, the sequence for applying a voltage to a driver IC is set in such a manner that Voff voltage is applied first and Von voltage later when the device is turned on, and Von voltage is applied first and Voff voltage later when the device is turned off.
If necessary, an LCD device includes a power sequence controlling circuit. Von voltage and Voff voltage are generated independently from each other in a conventional power sequence controlling circuit. A sequence of such voltages is controlled by time constants of a plurality of DC-to-DC converters 1 and 2 for outputting Von voltage and Voff voltage, or only by a time constant of Von voltage.
However, in the above-described sequence control method, Von voltage and Voff voltage are applied to a driver IC independently from each other. Thus, a relative time control for keeping the sequence is difficult to achieve. Specifically, the above-described conventional method allows a sequence control only when a power is turned on.
Accordingly, as shown in FIG. 2 the voltage applying sequence of the driver IC 3 is not followed correctly, which causes a latch up. This may result in a failure in driving an LCD panel 4.
In the meantime, a latch up may occur while controlling the voltage applying sequence, by failing to keep the voltage level applied to the driver ICs.
In more detail, after voltage VDD is applied in accordance with the normal sequence, voltage Voff of approximately xe2x88x927V and voltage Von of approximately 20V are applied to the driver IC as a reference voltage for controlling a TFT. However, a current may flow to the path for applying voltage Von or Voff before the voltages of Von and Voff are stabilized to the level of xe2x88x927V and 20V respectively. Thus, due to such a current, a voltage exceeding the scope of xe2x88x920.5V of Voff (and 0.5V for Von), a requisite for preventing latch up of the driver IC, is applied to the driver IC. As a result, an excessive current is generated to the CMOS (complementary metal-oxide semiconductor) circuit which constitutes the driver IC. Thus, the DC-to-DC converter is shut down due to an excessive current applied thereto, which impedes driving of the LCD module.
It is therefore an object of the present invention to apply a plurality of voltages to a driver IC for outputting a driving voltage of an LCD panel in accordance with a predetermined sequence by allowing the plurality of voltages to be dependent upon each other.
It is another object of the present invention to stabilize driving of the LCD panel by applying the voltages to the driver IC in accordance with a predetermined sequence.
It is still another object of the present invention to provide a normal operation of the LCD panel by stabilizing the driving voltage applied to the driver IC and preventing a latch up of the driver IC.
To achieve the above objects and other advantages, there is provided a power supply of an LCD including a first and a second DC-to-DC converters for converting a constant voltage and outputting a first and a second voltages which are different from each other, a switching device for switching the first voltage based on a level of the second voltage and outputting a converted third voltage and a gate driver integrated circuit (IC) for determining the second voltage as a turn-off voltage and the third voltage as a turn-on voltage and outputting a signal for driving an LCD panel.
The switching device consists of a switching element and voltage dividing resistances connected thereto. A pnp-type bipolar transistor or a p-type MOS transistor can be used as the switching element.
For the pnp-type bipolar transistor, a potential difference caused by the voltage dividing resistance is required to be set higher than those between an emitter and a base. For the p-type MOS transistor, the potential difference caused by the voltage dividing resistance is required to be set higher than a threshold voltage.
The voltage sequence according to the above-described constitution is controlled by applying voltages for turning on and off the LCD panel to a driver IC for outputting on and off signals for driving the LCD panel. In addition, the level of the turn-off voltage switches to control the level and time for applying the voltage for turning on the LCD panel.
Accordingly, when an external power is applied to the LCD panel, the voltage for turning on the LCD panel is applied after the voltage for turning off the LCD panel is applied to the driver IC. When the external power is turned off, the turn-on voltage level is removed prior to the removal of the turn-off voltage level and these sequences are controlled automatically.
The present invention may use as a latch up preventive device a first diode connected in forward direction to a portion of the driver IC to which the first voltage is applied, and a second diode connected in reverse direction to a portion of the driver IC to which the second voltage is applied.